The present invention relates generally to a waveform display system, and more particularly to a system for minimizing display distortion caused by changes in electron beam current of a cathode-ray tube (CRT) of an oscilloscope.
In an oscilloscope system, the electron beam of the CRT is deflected horizontally and vertically in response to X-axis and Y-axis deflection signals, respectively, while the CRT beam current is controlled (Z-axis), thus displaying waveforms or alpha-numeric character readout on the CRT screen. The readout is an alpha-numeric character display representing deflection factors or various measurement related information for measurement convenience. The CRT beam current is cut off during sweep retrace, readout blanking in the readout display mode, chop blanking in a so-called CHOP mode, etc. Also, the CRT beam current level is changed in an A intensified by B sweep mode wherein the intensity level is increased during one portion of the main or A sweep, thereby indicating the delayed or B sweep period. Such changes of the CRT beam current induce incidental deflections of the CRT beam due to minor misalignments of various elements of the CRT gun. As a result, undesired display distortion is caused as the CRT beam current changes such that display quality suffers, ground reference is uncertain, timing errors exist, display readout shifts with beam intensity, distortions exist during the A intensified by B sweep display mode, and vertical aberrations occur at the higher portions of the CRT operating curve. The effect is more pronounced in certain high performance CRTs with high beam currents and high deflection sensitivites such as CRTs including quadrapolar scan expansion electrodes or a scan expansion mesh electrode.